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AD8348
The level of the mixer output (or the output of the baseband
filter) can then be set by varying the setpoint voltage fed to
Pin 11 (VSET) of the AD8362.
Care should be taken to ensure that blockers—unwanted signals
in the band of interest that are demodulated along with the desired
signal—do not dominate the rms power of the AD8362 input.
This can cause an undesired reduction in the level of the mixer
output. To overcome this, baseband filtering can be implemented
to filter out undesired signals before the signal is presented to
the AD8362.
Figure 56 shows the effectiveness of the AGC loop in
maintaining a baseband amplifier output amplitude with less
than 0.5 dB of amplitude error over an IF input range of 40 dB
while demodulating a QPSK-modulated signal at 380 MHz.
The AD8362 is insensitive to crest factor variations and
therefore provides similar performance regardless of the
modulation of the incoming signal.
Figure 57 shows the schematic for a 100 Ω, fourth-order elliptic
low-pass filter with a 3 dB cutoff frequency of 20 MHz. Source
and load impedances of approximately 100 Ω ensure that the
filter sees a matched source and load. This also ensures that the
mixer output is driving an overall load of 200 Ω. Note that the
shunt termination resistor is tied to the source driving VCMO
and not to ground. This ensures that the input to the baseband
amplifier is biased to the proper reference level. VCMO is not
an output pin and must be biased by a low impedance source.
The frequency response and group delay of this filter are shown
in Figure 58 and Figure 59.
0
–10
–20
–30
140
130
–5.1dBm re 10k ?
QPSK
3
2
–40
–50
–60
120
1
–70
110
ERROR
0
–80
1
10
100
100
90
80
–1
–2
–3
50
45
FREQUENCY (MHz)
Figure 58. Baseband Filter Response
70
–55
–45
–35 –25 –15 –5
IFIP POWER INPUT (dBm, Z O = 200 ? )
5
–4
40
35
Figure 56. AD8348 Baseband Amplifier Output vs.
30
IF Input Power with AD8362 AGC Loop
BASEBAND FILTERS
Baseband low-pass or band-pass filtering can be conveniently
performed between the mixer outputs (IMXO and QMXO) and
the input to the baseband amplifiers. Consideration should be
25
20
15
10
5
1
2
given to the output impedance of the mixers (40 Ω).
C1
C2
4.7pF
8.2pF
0
1
10
FREQUENCY (MHz)
Figure 59. Baseband Filter Group Delay
100
R1
60 ?
L1
0.68μH
L2
1.2μH
C5
C6
R2
TO AD8362
INPUT IF AGC
LOOP IS USED
150pF
82pF
100 ?
IMXO
VCMO
IAIN
AD8348
Figure 57. Baseband Filter Schematic
Rev. A | Page 22 of 28
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